I've used them both extensively. In fact I own SystemWorks 2005 Premier.
I have encountered a number of problems using System Mechanic in addition to those mentioned in the previous posts.
1) I do not like its registry utility. I find the entries it selects for correction are obscure and potentially dangerous to delete.
2) Its duplicate file remover is also potentially dangerous and is almost a waste of space since you are repeatedly cautioned to not remove the duplicates it finds lest they be necessary for stable system function. How many users can say for sure whether a given number of duplicate files is fit for removal or essential to their PC's performance? Most people just do not have that kind of intimate knowledge of their system files.
3)The defraggger is slooowww.
4)The memory defragger is somewhat useless since PC memory should only rarely, if ever, be defragged.
5)The registry compactor is somewhat redundant as the windows registry seldom needs to be compressed.
The registry compressor feature puts, in my mind, an unnecssary and potentially harmful state as it rquires a reboot or two in order to complete its function. I am leary of software features that requires repeated reboots as I beleive it is hard on your PC and is indicative of lazy code writers.
If you want to compress your registry from time to time then get KSL's Registry Compressor, for example.
In contast, SystemWorks has two safe and user friendly registry utilities which require no guesswork or investigation on the part of the user. You just click Start and NSW does the rest.Speedisk is much quicker than Sytem Mechanic's defragger and is always favorably reviewed. CleanSweep is fast effective and safe.The files it deletes are always safe to delete and best of all it does it all for you, no decision or wondering on the part of the user. As with System Mechanic, NSW comes with a variety of system settings, tweak and optimization utilities. In fact, NSW comes with many more of these than System Mechanic.
Lastly, NSW comes with Norton antivirus. With System Mechanic you must pay extra for an antivirus(Kaspersky) and firewall(also Kaspersky). System Mechanic does come with a proprietary spyware/adware scanner called Spython. I have no idea how effective it is or is not save to say it does its scan in a remarkably short time bringing into question its thoroughness.
The only other PC utility suite(s) I would recommend to you is V-Com's System Suite and Fix-it Utilities, neither of which I have used but have heard many good things about.
SystemWorks has its detractors to be sure. Many complain of system drag and poor technical support, for example. A lot of Symantec programs take up exceptionally large amounts of space and consume unusually high percentages of memory. Symantec, for some reason, includes its liveupdate feature separately from its programs, which means even more space taken up by already large Symantec files.It is also an involved process getting Symantec products off your machine.
I could go on,but I hope this is enough to help you make an informed decision.